Cool air from small opening and hot air from wide-open mouth

If you blow on your hand with your mouth open, your breath will be warm and if you do it with reducing the opening of your lips, your breath will be appreciably cooler. This is because expanding air is always cool and compressed air is always warm. A molecule picks up speed when it is hit by another molecule that approaches with a greater speed. But when a molecule collides with one that is receding, its rebound speed is reduced. The same idea applies to a region of air that is expanding: Molecules collide, on average, with more molecules that are receding than with molecules that are approaching. Therefore, in case of expanding air the speed of molecules decreases and thus cools the air. But in case of compressed condition, the molecule collide with more molecules that are approaching than with molecules that is receding. So, in case of compressed air the speed of molecules increases and thus warms the air.      

How do fishes survive in the coldest of winter?

When ice water freezes to become solid ice, its volume increases and its density is lowered. That’s why ice floats on water. If water were most dense at 0°C, it would settle to the bottom of a pond or lake. Water at 0°C, however, is less dense and floats at the surface. The water of a pond or lake freezes from the surface to the downward direction. The temperature at the bottom of an ice covered pond is 4°C, which is very much warm for the fishes that live there. More importantly, it is not possible to cover very deep ponds with ice even in the coldest of winters. This is because all the water must be cooled to 4°C before lower temperatures can be reached. For deep water, the winter is not long enough to reduce an entire pond to 4°C. This is because water has a very high specific heat capacity and poor ability to conduct heat. That is why the temperature in a cold region remains at a constant 4°C year-round.           

Why does water take longer times to warm than sand?

Specific heat capacity plays an important role here. It is defined as the amount of heat needed to change the temperature of a unit mass of the substance by 1 °C. It is also known as thermal Inertia which means the resistance of a substance to any change in its temperature. A relatively small amount of water absorbs a large quantity of heat for a correspondingly small temperature rise as water has a very high capacity for storing energy. Water also takes a long time to cool as it has a tendency to resist changes in temperature. For its higher heat capacity, it takes longer time to warm in presence of hot sunlight and longer time to cool on a cold night. Sands heat capacity is very low. Therefore, it warms quickly in presence of sunlight and cools quickly at night.   

Reason of dropping a coin straight into the glass by removing a card suddenly?

According to Newton's law of motion, If no external force is applied on a body, a stationary body will remain stationary and a moving body will continue to move without changing its speed. So a body continues to do whatever it happens to be doing unless a force is exerted upon it. In the absence of net forces, a moving body tends to move along a straight-line path indefinitely. It is the property of the body to resist changes in motion that is widely known as inertia. To show the property of that body experimentally, a card is put on the top of a glass with a coin on the upper face of the card. Now If we accelerate the card with a force, the coin will drop straight into the glass. As we said earlier, a stationary body continues to remain stationary due to inertia. As the coin was initially in stationary state, after removing the card very rapidly, it wanted to continue its state of rest. That is why it dropped straight into the glass.

How is it possible to lie without harm on a bed of nails?

We know that pressure is a force divided by area over which the force is exerted. When a person lies on a bed of nails, the force equal to the weight of that person acts on the surface of the bed containing nails. Here, the weight is distributed over hundreds of nails that makes the pressure at the tip of each nail safely small. If the same person stands on his feet, he will surely feel the sharpness of the nails because in this case the area of  contact is very small. So, the weight will be distributed over a few nails and hence the pressure will be relatively large.   

What is the reason of cooler air coming out of a bursting tire?

We know from the ideal gas law that pressure increases proportionally with the increase of temperature when volume is held constant for a given mass. As the air inside the tire is in compressed state, the temperature inside the tire is high. When the tire bursts, air inside the tire comes out very fast causing a sudden expansion and decrease in temperature. Therefore, the air is relatively cooler than the surroundings. Blowout of tire occurs when the location of tearing of the tire is incapable of containing the pressurized air and the escaping air causes further tear of the tire.

How glass is broken with sound?

When any object composed of an elastic material is disturbed, it will vibrate at its own special set of frequencies, which together form its special sound.The frequency is the natural frequency of that object. The tendency of one object to force another object into vibrational motion is called forced vibration. If an unmounted tuning fork is struck, the sound it makes is faint. Hold the base of the fork on a table top, and the sound is relatively loud. This is because the table is forced to vibrate, and its larger surface sets more air in motion. The table top becomes a sounding board, and can be forced into vibration with forks of various frequencies. When the frequency of forced vibrations on an object matches the object's natural frequency, a dramatic increase in amplitude occurs. This phenomenon is known as resonance. A glass has a natural frequency. Now if the pitch of the sound creating forced vibration match the resonant frequency of glass the vibrating air will cause the glass vibrating too. If this occur with sufficient volume, right frequency and high enough amplitude, this will cause exceed the strength of the glass to resist the vibrations. When the sound gets too loud for the glass to vibrate, it breaks the glass.